FCP7 has no trouble with ProRes 422.
FCP7 can handle all 6 flavors of ProRes. (XQ requires Mountain Lion or later.)

The designation "APCN" is the work of fourcc.org. They have devised 4-byte code names for many video codecs in use. Maybe they've named all 6 flavors of ProRes, maybe not. These code names are of no use to this discussion, especially since you're converting APCN to ... duh?

Dennis Couzin
Berlin, Germany

[Note added 14 June 2018:
The fourcc.org namings are both a way to standardize video codecs and to liberalize them. Some codec makers who have approximated Apple ProRes 422 without daring to call it by that name, can now call it "APCN".

The word "codec" abbreviates "code-decode". An encoder and a decoder are two essentially separate programs. Apple makes the ProRes decoders easily available, in order for ProRes to be widely playable. But Apple restricts the ProRes encoders. A list of licensees is at [support.apple.com]. One knowledgeable about video compression, given a decoder, can reverse engineer a corresponding encoder. For ProRes, it probably won't be exactly the Apple encoder. It probably will be a bit worse, meaning the same source video will make a bit worse looking ProRes (at the same bit rate). But it could conceivably make a better looking ProRes (at the same bit rate) than the Apple encoder does.

I think MetalGuru's intention was to write:
I'm converting 23.976 APCN (like ProRes 422) to 23.976 ProRes 422 for use in FCP7 (FCP7 doesn't like APCN).

I don't know why FCP7 refused MetalGuru's APCN. It's not just a matter of trademarked names. I used a binary editor to blot out the explicit (ASCII) names "Apple" and "ProRes" in a ProRes 422 .mov file. FCP7 took it, and recognized it as ProRes 422.]

I've been supplied 23.976 ProRes files, when viewed in QT they shiow "apcn" and won;'t import properly into FCP7. My solkution was to re-convert to Prores using Compressor,but as I said, even when leaving all settings identical, Compressor is duplicating frames.

However, if I run the sames files through EditReady, I get perfect FCP7 ready files.